| Hence arises the natural illusion which induces us to believe that these lines proceed from an object which lies out of the sphere of empirical cognition, just as objects reflected in a mirror appear to be behind it. |
| But this illusion--which we may hinder from imposing upon us--is necessary and unavoidable, if we desire to see, not only those objects which lie before us, but those which are at a great distance behind us; that is to say, when, in the present case, we direct the aims of the understanding, beyond every given experience, towards an extension as great as can possibly be attained. |
| If we review our cognitions in their entire extent, we shall find that the peculiar business of reason is to arrange them into a system, that is to say, to give them connection according to a principle. |
| This unity presupposes an idea--the idea of the form of a whole (of cognition), preceding the determinate cognition of the parts, and containing the conditions which determine a priori to every part its place and relation to the other parts of the whole system. |
| This idea, accordingly, demands complete unity in the cognition of the understanding--not the unity of a contingent aggregate, but that of a system connected according to necessary laws. |
| It cannot be affirmed with propriety that this idea is a conception of an object; it is merely a conception of the complete unity of the conceptions of objects, in so far as this unity is available to the understanding as a rule. |
| Such conceptions of reason are not derived from nature; on the contrary, we employ them for the interrogation and investigation of nature, and regard our cognition as defective so long as it is not adequate to them. |
| We admit that such a thing as pure earth, pure water, or pure air, is not to be discovered. |
| And yet we require these conceptions (which have their origin in the reason, so far as regards their absolute purity and completeness) for the purpose of determining the share which each of these natural causes has in every phenomenon. |
| Thus the different kinds of matter are all referred to earths, as mere weight; to salts and inflammable bodies, as pure force; and finally, to water and air, as the vehicula of the former, or the machines employed by them in their operations--for the purpose of explaining the chemical action and reaction of bodies in accordance with the idea of a mechanism. |
| For, although not actually so expressed, the influence of such ideas of reason is very observable in the procedure of natural philosophers. |
| If reason is the faculty of deducing the particular from the general, and if the general be certain in se and given, it is only necessary that the judgement should subsume the particular under the general, the particular being thus necessarily determined. |
| I shall term this the demonstrative or apodeictic employment of reason. |
| If, however, the general is admitted as problematical only, and is a mere idea, the particular case is certain, but the universality of the rule which applies to this particular case remains a problem. |
| Several particular cases, the certainty of which is beyond doubt, are then taken and examined, for the purpose of discovering whether the rule is applicable to them; and if it appears that all the particular cases which can be collected follow from the rule, its universality is inferred, and at the same time, all the causes which have not, or cannot be presented to our observation, are concluded to be of the same character with those which we have observed. |
| This I shall term the hypothetical employment of the reason. |
| The hypothetical exercise of reason by the aid of ideas employed as problematical conceptions is properly not constitutive. |
| That is to say, if we consider the subject strictly, the truth of the rule, which has been employed as an hypothesis, does not follow from the use that is made of it by reason. |
| For how can we know all the possible cases that may arise? |
| some of which may, however, prove exceptions to the universality of the rule. |
| This employment of reason is merely regulative, and its sole aim is the introduction of unity into the aggregate of our particular cognitions, and thereby the approximating of the rule to universality. |